1. Field of the Invention
The invention relates to a starter unit for an internal combustion engine having a starter and a flywheel which can be coupled to or disengaged from the crankshaft of the engine with at least one clutch.
2. Prior Art
This type of starter unit is described in Patent Abstracts of Japan, Vol. 007, No. 199 (JP-A-58 098 658), in which a starter is connected to the engine crankshaft via a pinion/toothed ring transmission.
For a starter system, there are essentially three main types of apparatus:
1. Conventional electrical starter.
Such a design, because of the relatively brief attainable total service life of the starter motor and in particular its pinion, its toothed ring, and its override clutch, as well as for noise reasons no longer leads to an embodiment with a promising future.
2. Starter-generator with direct starting.
Here the starter-generator is solidly connected to the crankshaft. In starting, it acts as the starter motor, which is supplied by a battery, while during operation it acts as a generator for charging the battery.
Because of the necessary starting energy and starting power (approximately 150 Nm to 200 Nm of cold-starting torque for a medium-priced automobile) it is not feasible to use a starter generator with a typical 12 to 24 V lead starter battery. Furthermore, the electric motor of the starter-generator would have to be enlarged by more than a factor of 3 compared with what is needed for the generator power, which among other effects would also lead to problems in terms of space. For starting an engine at operating temperature, such a starter-generator would be unambiguously oversized.
3. Starter-generator with impulse starting.
For impulse starting, with the engine (and transmission) disengaged, the flywheel is first rotated up to a cranking rpm. As the clutch rapidly closes, the flywheel then starts up the engine with rotational energy.
Recent vehicle concepts, optimized in particular with regard to environmental protection and fuel consumption, require the engine to be turned off at traffic lights; the current term for this is "start-stop mode". Moreover, the engine should be either merely disengaged, or preferably turned off entirely when the engine is in an overrunning phase; the latter is accomplished by what is known as an "automatic inertia utilization transmission".
An application of this modern technology, however, means that the number of starting cycles is increased by a factor of about 10, so that a starter that earlier was designed for 40,000 starting cycles not would have to withstand from 400,000 to 600,000 starting cycles. On the other hand, for reasons of comfort and also because an ever-greater number of electrically actuated accessories are being used, modern vehicles have a generator power that is higher by up to a factor of 5 (previously, 1 to 1.5 kW, and in future 5 kW and more). If the automatic inertia utilization transmission turns on frequently, still other problems arise in supplying the on-board electrical system.
In an embodiment according to German Patent DE 30 48 972 C2, the problem of generator power is solved in part by a generator installation on the crankshaft, between the engine and the flywheel; if a manual transmission is used, two clutches are then needed, which are advantageously disposed in front of and behind the flywheel. With such an arrangement, it is then possible with an automatic inertia utilization transmission to generate generator power.
Such an "impulse start" is suitable there for normal cold starts. For re-starts, however, of the kind done with the engine warm at a traffic light or at the end of an overrunning phase, it is unfavorable because of the time needed.
The same problem also exists in a design of the kind described in German Patent Disclosure DE 29 17 139. In this known starter unit, the centrifugal mass of the crankshaft can be engaged and disengaged, and on slowing down, braking or in overrunning the engine can be allowed to keep running by interrupting the drive train with a minimum rpm, such as the idling rpm. In brief stops, for instance when stopped at a traffic light, the drive train is also interrupted, but then the engine is turned off as well, while the flywheel continues to rotate and is then re-engaged via the clutch to restart the engine. It is also possible if the flywheel rpm has dropped too far to use an electric motor to increase the flywheel rpm again.
Overall, until now, in a crankshaft starter-generator concept, either an excessively high starting moment for the cold start, as a direct start system, was definitive for designing the electrical machine and various other electrical components, so that overall an appropriate compromise would not be found, or if a classical starter or an impulse starter was used, the demands for the re-starting with regard to starting time, low noise level, wear and service life, for the high demanded number of 400,000 cycles and more, could not all be attained.